The invention relates to a process and device for monitoring the quality of yarns.
A device for yarn-clearers is known from CH-477573, for example, with the aid of which it is possible to perform monitoring of the quality of yarns by cutting out defects which exceed certain limit values. In that instance, a defect is characterized by the fact that it exceeds, or falls below, the average diameter of the yarn over a finite length. Certain defects are cut out by a knife and the yarn is then spliced again. In order to activate a cutting knife, a so-called clearing limit is fixed, against which all defects are measured. Those defects which exceed the clearing limit are cut out by the cutting knife. The clearing limit is composed of points, each of which is characterized by a certain deviation from the average diameter, and by a certain length of a portion of yarn. Known clearing limits have a continuous course or else a discontinuous course (with discontinuities). However, they usually have the effect that deviations from the diameter which are major ones but of only brief length are not cut out, whereas minor deviations of great length are cut out. It is known practice to locate such clearing limits over a classifying field, under which circumstances the class limits and the clearing limit do not always coincide. However, such a clearing limit always divides the space or an area into two regions: a region outside the clearing limit for intolerable defects and a region inside the clearing limit for tolerable defects.
A disadvantage of these known devices can be seen, for example, in the fact that, in addition to the irregularities in the yarn per se, foreign substances which are spun into the yarn and bring about a change in diameter, are cleared out in the same way as ordinary thick or thin points in the yarn. This means that it is not possible to distinguish between the various foreign substances which are encountered, such as foreign bodies, shell parts, fibers of other colors, hairs, knots, etc. Thus, it is likewise not possible to selectively cut out individual types of foreign substances. Known devices are equipped with sensors which make it possible to detect the diameter or the mass of a portion of yarn and, from it, to monitor the uniformity of the mass or diameter, viewed over the length of the yarn, or even to identify foreign substances in the yarn in a general manner. An electrical signal, which is correlated with a portion of yarn, is usually produced in the sensor. However it is difficult and unreliable to deduce therefrom whether a certain foreign body is now contained in the portion of yarn. It is possible to identify the presence of a foreign body, but impossible to determine its nature.
There is therefore no possibility of distinguishing between yarn and foreign substance, or between various kinds of foreign substances in the yarn. Shell parts in the yarn are usually removed from the latter by further processing, for example chemical aftertreatment for bleaching purposes, as they can be very successfully identified later in the woven cloth. They are often immediately conspicuous, or else become apparent through the fact that they do not take on color, or do so only inadequately, during the dyeing process.